Mirror without deleting or adding new material
I assume you mean that you edited the Rig. In that case you can do this:


In the tool shelf: Vertical Options Tab enable X-Mirror
In your Armature, in Edit Mode: select all bones which you have modified (similar like in the image)
press "g" followed by RETURN will instantly adjust the other side of your rig.


Note: This works only if the Rig's symmetry plane is YZ (character looks along Y axis) and when the Bones are named according to Blender's naming conventions, see http://wiki.blender.org/index.php/Doc:2.6/Manual/Rigging/Armatures/Editing/Properties